1. Field of the Invention
The present invention is broadly concerned with boiler assemblies and methods of operation thereof giving enhanced pollution removal. More particularly, the invention is concerned with such assemblies and methods wherein fresh sorbent material is injected into the assembly and reacts with flue gas pollutants; the flue gas and entrained sorbent are then passed through a recirculation reactor where sorbent is recovered for reinjection into the system. In preferred forms, the assemblies include a flue gas recirculation reactor such as a multi-clone device and a downstream particulate separator/collector such as a bag house or electrostatic precipitator. A constant mass or volume flow recirculation reactor recycle rate is established at the reactor to maximize the efficiency of the system, and an optional, secondary, constant mass or volume flow recirculation rate apparatus may be employed between the collector and the reactor. The invention provides a high degree of pollution abatement with low cost operation.
2. Description of the Prior Art
In fuel-fired boiler assemblies, and particularly coal-fired power generating plants or other industrial processes, combustion products include many compounds having an adverse influence on boiler operation or are environmentally undesirable and subject to government regulation. Such compounds include sulfur oxides (SOx), nitrogen oxides (NOx), hydrochloric acid, and heavy metals such as Hg, As, Pb, Se, and Ca. Additionally, a significant number of nations, including the European Union and Japan, have taken steps to further limit the emissions of carbon dioxide.
In order to meet environmental limitations affecting the discharge into the atmosphere of the most prevalent of the widely regulated compounds, sulfur dioxide, combustion products from these plants and processes are commonly passed through flue gas desulfurization (FGD) systems. The treatment of flue gases to capture sulfur dioxide is often accomplished in lime or limestone-based wet, semi-dry and/or dry scrubbers where lime and limestone slurries and/or dry sorbents contact the flue gases before they are discharged to the atmosphere. The sulfur oxides are thereby chemically converted into insoluble compounds in the form of sulfites or sulfates. The sulfur oxides are thus converted into less environmentally harmful compounds which are either disposed of in landfills or treated and sold as marketable chemicals.
The SO3 emission problem has been addressed chemically using a variety of alkaline chemicals (wet and dry) that are injected into the system at many different points in the flue gas flow path. Lime or limestone injected into the high temperature region of the boiler can also be effective in capturing the SO3, but the commercial materials that are generally utilized tend to magnify boiler deposit problems and increase the quantity of particulates that can escape. Sodium compounds, such as the bisulfite, carbonate, bicarbonate and sodium sesquicarbonate (Trona) compounds have also been injected into the flue gas stream and are effective in SO2 and SO3 capture. Commercially available, but relatively expensive, oil-based magnesium additives can be effective in SO3 capture. In that regard, one of the most effective chemical techniques for controlling both ash-related fouling in the boiler, and also the corrosion and emission problems associated with SO3 generated in solid-fueled boilers, is the injection into the upper region of the boiler of oil slurries of MgO or Mg(OH)2. That technology was originally developed for use with oil-fired boilers in which the magnesium-based oil suspension was usually metered into the fuel. It was later applied to coal-fired boilers. The most widely accepted mode of application of such additives today is by injection of slurries of MgO or Mg(OH)2 into the boiler above the burners and just below the region at which a transition from radiant heat transfer to convective heat transfer occurs.
Another approach to SO3 capture involves the use of so-called “overbased” organic-acid-neutralizing additives of the type that are included in motor oils and as fuel oil combustion additives. Those additives are actually colloidal dispersions of metallic carbonates, usually magnesium or calcium. When burned with the fuel, they are effective at near stoichiometric dosage in capturing SO3 and in mitigating ash deposits caused by vanadium and/or sodium in the oil. The colloids are stabilized by carboxylic or sulphonate compounds and are known to provide mostly particles in the Angstrom range. Though very expensive, the “overbased” compounds are widely used at low dosages to capture vanadium in heavy-oil-fired combustion turbines.
In addition to oil-based slurries, Mg(OH)2 powders and water-based slurries have also been utilized as fireside additives in boilers, but because of their generally coarser particle size they are less efficient in capturing the SO3. Water slurries of MgO have also been injected through specially modified soot blowers installed on oil and Kraft-liquor-fired boilers, in which they moderated high temperature deposits but had only a nominal impact on SO3-related problems because of an inability to apply the chemicals continuously.
In addition to limitations on SOx emissions, regulations aimed at controlling mercury emissions from coal-fired boilers have been promulgated by regulatory authorities, and regulations applicable to other toxic metals are anticipated eventually. A considerable amount of research aimed at finding practical techniques for capturing such toxic metals has shown that high-surface-area solids can capture a significant portion of mercury by adsorption, if the mercury is in an oxidized form rather than in an elemental form. Oxidants, either added to or naturally present in the fuel, such as chlorides, can facilitate the oxidation. Although high-surface-area lime can be effective in mercury capture, the usual commercial products can result in operational problems in the form of ash deposits and increased stack emissions. The most widely accepted way to achieve mercury capture has been the injection of expensive activated carbons in the cooler regions of the boiler gas path.
In addition, a variety of bromides and related compounds (e.g., iodates) have been used for control of mercury in boiler flue gasses, alone or in combination with activated carbon, clays, zeolites, and fly ash.
References describing the use of a plurality of inorganic carbonates, hydroxides and oxide compounds for boiler pollution abatement include: US Publications Nos. 2008/0286183; 2008/0233028; 2006/0005750; U.S. Pat. Nos. 6,528,030; 4,983,187; 4,824,441; 4,801,438; 4,783,197; 4,562,054; 4,522,626; 4,515,601; 4,226,601; 4,192,652; 4,148,613; 3,970,434; and German Patent No. DE 3,317,504.
Prior references which disclose the use of at least two carbonate, hydroxide or oxide compounds include: US Publications Nos. 2008/0060519; 2006/0034743; U.S. Pat. Nos. 7,276,217; 7,013,817; 5,505,746; 5,458,659; 5,350,431; 4,555,390; 4,280,817; 4,274,836; 4,092,125; and 4,055,400.
References teaching the use of single carbonates, hydroxides or oxides include: US Publications Nos. 2008/0279743; 2004/0202594; 2002/0050094; U.S. Pat. Nos. 7,374,590; 7,056,359; 5,368,617; 4,886,519; 4,574,045; 4,516,980; 4,423,702; 4,395,975; 4,305,728; and 4,302,207.
References describing the use of bromide compounds for removal of mercury with or without other inorganic components include: US Publications Nos. 2008/0182747; 2008/0134888; 2008/0121142; 2008/0115704; 2007/0180990; 2006/0205592; 2006/0204418; 2006/0185226; 2004/0086439; 2004/0003716; U.S. Pat. No. 6,878,358; PCT Publication No. WO 2006/101499; US Publications Nos. 2008/0207443; 2008/0127631; 2006/0210463; 2003/0161771; U.S. Pat. Nos. 4,859,438; 4,663,136; 4,233,175; and 4,115,518.
The longstanding pollution abatement technologies employed with fossil fuel-fired boiler assemblies, while useful to a certain degree, do not achieve the highest degree of pollution control. Moreover, the prospect of increasingly stringent government pollution regulations makes it imperative that improved technologies be provided. It is the aim of the present invention to remedy this problem.